When a vehicle turns, the lateral acceleration produces a moment, orientated with respect to the longitudinal axis of the vehicle, causing a change in the seating of the vehicle body: the torque rods of the wheels positioned inside the bend are subject to a different load from that to which the torque rods situated on the outside of the bend are subject. Relative to the body, the wheel supports are displaced by a different amount along the (virtually vertical) axes of the torque rods. Relative to the axis passing through the wheels of a set of wheels, the body is inclined by an angle known as the angle of roll.
In order to reduce the roll of a vehicle, it is known to provide the front set of wheels and/or the rear set of wheels with a passive anti-roll device. This consists of a generally U-shaped torsion bar, including a rectilinear central portion orientated in the direction of the axis of the wheels of the set of wheels, and two arms situated at each of the ends of the said central portion and generally extending perpendicularly to this. The central portion is connected to the body by a connection of pivot type permitting rotation of the bar about the axis of the central portion. The free ends of the arms are respectively connected to each of the wheel supports (for example to the hub carrier or the lower portion of the torque rod of each of the wheels).
In a bend, the difference in the amount of vertical displacement of the wheel supports deforms the torsion bar so that an angle of torsion appears between its arms. An elastic return torque is thus created, opposing the movement of the wheel supports and reducing the roll motion of the body. No energy is contributed to the anti-roll device, which is then qualified as passive.
Although the roll of the vehicle and, consequently, passenger comfort, is improved in bends the torsion bar reduces the isolation of the body relative to the road. In a straight line, a shock caused by an unevenness on the road will be felt by the passengers. Moreover, the handleability of the vehicle is modified.
Active anti-roll devices are known. For example, the document U.S. Pat. No. 6,425,585 discloses an electromechanical active anti-roll device. In this device, the U-shaped anti-roll bar is formed of two half-bars connected to each other at the central portion of the anti-roll bar by means of a coupling mechanism. This type of device is qualified as active as the coupling mechanism is able to generate and apply a counter-torque between the two half-bars. It is necessary to contribute energy to the coupling mechanism to generate this counter-torque.
Electromechanical semi-active anti-roll devices are also known which permit selective coupling or decoupling of the first and second half-bars. No pre-stress is then applied between the two half-bars, and such semi-active antiroll devices do not require energy contribution to create a counter-torque. The document EP 1 157 865 discloses a series of embodiments of an electromechanical semi-active anti-roll device including clutches, brakes or the equivalent permitting coupling/decoupling of the two half-bars as a function of a command emitted by a control unit on the basis of measurements of the displacement of the torque rods taken by appropriate sensors. The coupling/decoupling can for example be of the “on/off” type or of the “proportional” type so that an impact on one wheel is not transmitted to the other wheel. In certain embodiments of this document, it is necessary to contribute a large amount of energy to maintain the device in a coupled or decoupled state. In other embodiments, it is necessary to contribute a large amount of energy to switch the device from a coupled state to a decoupled state or vice versa. Moreover, it is necessary for the vehicle to be in a state in which the torsion bar is not twisted to pass from a totally or partially decoupled state to the coupled state, to prevent the anti-roll bar from continuously applying a return torque even when this is not necessary.
The document EP 0 974 477 describes an anti-roll bar including a semi-passive coupling mechanism of variable stiffness. The coupling mechanism comprises a fluid coupling filled with electrorheological fluid. For this mechanism to present a given stiffness, it is necessary to continuously apply an electrical field of corresponding amplitude to the electrorheological fluid. This mechanism therefore requires a large amount of energy. Moreover, fluid coupling must present meanders and be properly sealed. It is therefore bulky and expensive.